Changes from Marc to implement fuse_get_context():
* an XS function to get the data
* a mention of it in Fuse.pm so it can be exported properly
* a pretty ugly hack to example.pl, which is how I tested it.

Changes from Mark Glines in preparation for 0.07
- Remove the FUSE_DEBUG constant; we never actually implemented
it to begin with.
- "make test" now uses the version of Fuse you've just built,
not the one installed in /usr/lib/perl5.
- getattr test now allows blksize to vary between host and fuse
fs, as this is not a bug.
- Add experimental support for threading. The following minor
API changes accommodate this:
- The nonexistent (yet documented) "unthreaded=>1" attribute
has been replaced with the "threaded=>1" attribute, and this
time it actually exists.
- Symbolic refs like "main::e_getattr" are now allowed for
callbacks, because threaded mode needs to share() the
callbacks, yet perl 5.8.7 does not allow share()ing code
refs yet. Direct code-refs are still supported as much
as possible (currently, non-threaded mode).
- testsuite uses a multithreaded loopback.pl, when available.
- Update docs accordingly. Update examples accordingly.